The present invention relates to a communications system for communicating data, which for example may be speech data, encrypted speech data, data from instruments or facsimiles, which data has a classification associated with it. In particular, but not exclusively, the invention relates to a communications system for use on board an aircraft, which communications system segregates clear and secure information.
"Secure internal communication system" is defined for the purposes of this specification as a communication system intended to carry at least some classified (secure) material, the system being internal in the sense that access to the system is restricted, as for example where the system is located on board a military aircraft.
On board some aircraft, especially military aircraft, it is necessary to be able to communicate both "secure" and "clear" data between two or more points on that aircraft. It is also necessary and important that communication can also be established quickly between one or more points on the aircraft and a point external to the aircraft, normally by means of radio apparatus. Satisfying both these requirements reliably results in a complex communications system in order to ensure that secure data cannot accidentally be transmitted externally, or to an unintentional recipient aboard the aircraft itself.
In certain military aircraft, for example advanced early warning aircraft (AWACS), there may be up to thirty or more interfaces between the communications system and equipment or crew members, which interfaces need to communicate both clear and secure information both within the aircraft and also to external sources via a radio link. FIG. 1 of the attached drawings schematically shows the arrangement adopted in such communications systems. This comprises a secure communication network 1 and clear communication network 2 to which a plurality of interfaces 5 to 7 are connected. Interfaces 3 to 4 may transmit speech data to or from a member of the air crew or provide information to or from instruments aboard the aircraft. Depending on the source of the information, this will either be routed via the secure network 1 or clear network 2. In this way, information on the secure network 1 is isolated from radio 7. Some crew members have the option of transmitting information either by secure network 1 or clear network 2, and this is typically achieved by means of a switch which indicates over which network the operator wishes to transmit. Secure information from any source which is to be transmitted externally via radio 7 is placed on the secure network 1 and addressed to an encryptor 6. The encryptor encrypts the data before transmitting it to the radio 7 by clear network 2. Similarly, any encrypted data received via the radio is decoded by encryptor 6, and the decoded data transmitted to its destination by secure network 1.